1. Field of the Invention
This invention relates to devices and compositions comprising relaxed heterojunctions, and methods of use and fabrication thereof.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
The alloy system (Al,Ga,In)N, for example, is a direct band gap system with a band gap ranging from 6.1 eV, for AlN, to 0.7 eV, for InN. The lattice mismatch between AlN and InN is, however, as large as 13%, with 10% mismatch between GaN and InN. Expanding the operation range of (Al,Ga,In)N devices into the green, yellow and red range of the electromagnetic spectrum is therefore complicated by an extremely large lattice mismatch when GaN is considered as the substrate. To date, however, only bulk GaN substrates are available, and only a few attempts have been undertaken to fabricate thick InGaN layers on GaN as alloy substrates, because of difficulties in the growth of InGaN using the typical substrate growth method, Hydride Vapor Phase Epitaxy (HVPE). In addition, certain applications, for example multi junction solar cells, require vertical stacking of relatively thin layers with large differences in their lattice constant. For all lattice mismatched systems, the critical thickness of a mismatched layer, representing the maximum thickness for the deposition of a defect free layer, is inversely proportional to the lattice mismatch [1]. For In0.3Ga0.7N on GaN, for example, the critical thickness was estimated to be below 3 nm. In addition, when grown in the typical c-direction, the large lattice mismatch between GaN and InGaN layers is accompanied by the existence of large polarization fields in the crystal, which result in electron hole separation in InGaN/GaN quantum wells, reducing the recombination probability of excitons. Since, in the typical Ga-polar InGaN/GaN heterojunction light emitting devices, the internal polarization field is directed in the opposite direction than the externally applied electric field, further problems arise, in particular for solar cell applications. In all cases, a reduction in the lattice mismatch between the active area of the device and the surrounding layers would greatly expand device design opportunities.